Conductive inks or pastes are used to form metal contacts, such as silver gridlines and bus bars, on the surface of substrates such as silicon. Such substrates can be used in solar cells or photovoltaic cells which convert solar energy to electrical energy when photons from sunlight excite electrons on semiconductors from the valance band to the conduction band. The electrons which flow to the conduction band are collected by the metal contacts. Crystalline silicon solar cells in today's industry are typically coated with an anti-reflection coating to promote light adsorption, which increases cell efficiency. However, the anti-reflection coating also works as an insulator by preventing electrons from transferring from the substrate to the metal contact. Solar cells are typically covered by the anti-reflection coating before a conductive ink is applied. The anti-reflection coatings often comprise silicon nitride, titanium oxide or silicon oxide.
Conductive inks usually include a glass frit, conductive species and an organic medium. The conductive species, typically metal particles such as silver, provide conductive properties and function as current collectors after formation of the metal contacts. To form the metal contacts, conductive inks are printed onto a substrate. The substrate is then fired at a temperature in the range of about 650° C. to about 950° C. A sintering aid is in most instances because the firing temperature is lower than the eutectic point of silver and silicon, and the silver melting point. In addition, solar cells are typically covered by an anti-reflection coating before a conductive ink is applied. The conductive ink should penetrate the anti-reflection coating disposed on the substrate to form metal contacts having ohmic contact with the substrate.
Conductive inks incorporate glass frits to aid with sintering metal particles to a substrate and to promote adhesion and ohmic contact between the formed metal contact and the substrate. Depending on the formulation, glass frits can liquefy upon firing at a temperature between about 300° C. and 600° C. When the glass frit liquefies, it tends to flow toward the interface between the metal particles and the anti-reflection coating disposed on the substrate. The melted glass dissolves the anti-reflection coating materials as well as some of the silver and substrate. Once the temperature decreases, the molten silver and the melted or dissolved substrate recrystallize through the liquid phase. As a result, some of the silver crystallites are able to penetrate the anti-reflection layer and form ohmic contact with the substrate. This process is referred to as “fire-through” and facilitates a low contact resistance formation and a stronger bond between silver and the substrate. If a selected frit too aggressive, the substrate can be contaminated thereby degrading the solar cell performance. Selection of appropriate frits or mixtures of frit-precursors helps to avoid such contamination and to achieve good cell efficiency.
Accordingly, there is a need for a conductive ink which improves the series resistance in a photovoltaic cell, aids in sintering and also has the capability of promoting adhesion and ohmic contact of metal contacts and the substrate through anti-reflection coatings.